Hi iokinpardo,
Thank you for your purchase.

That is normal behavioral of the plugin for canceled or rejected appointments, because when customer  cancel the appointment it will become available for other customers. For Approved appointments will be removed time slots if they are not group appointments (if maximum capacity is more than one)

Same thing if  employee do that, but if you need to make those time slots not available then you can set Days off for that day, or if yo are using Google synchronization then you can make events in Google calendar and then turn on option Remove Google Calendar Busy Slots. Enable this option if you want to remove the busy slots in your employees’ Google Calendars from their work schedules in Amelia. When enabled, time slots from the Amelia Calendar will be removed whenever there is an event in the Google Calendar. 

Best regards.